The invention relates to an article having at least a modified mullite coating. The invention further relates to a silicon-containing substrate having at least a modified mullite coating. The invention further relates to a silicon-containing ceramic substrate having a modified mullite coating and at least one additional layer of material.
Silicon-containing materials have been proposed for structures used in high temperature applications, such as in heat exchangers and advanced internal combustion engines. For example, silicon-based composite ceramics have been proposed as materials for applications in combustors for commercial airplanes. However, these ceramic materials exhibit poor oxidation resistance in reducing atmospheres and in environments containing salts, water vapor or hydrogen. Hence, it is necessary to apply environmental barrier coatings to the silicon-containing materials to provide protection from environmental attack at elevated temperatures and to apply thermal barrier coatings to extend the life at elevated temperatures.
Mullite has been proposed as a material for environmental barrier coatings as well as thermal barrier coatings on silicon-containing materials. Mullite exhibits low thermal conductivity. It has low density and a high melting point. However, mullite coatings tend to develop cracks perpendicular to substrates and through the thickness of the coating. These cracks are detrimental to the functions of the mullite coating because they serve as transport paths for corrosive species causing severe oxidation and corrosion at the interface between the coating and substrate. Additionally, cracks in the coating concentrate stresses. The cracks apply shear and tensile forces on the substrate to cause substrate fractures.
Since the crack openings increase with increasing distance from the mullite substrate interface, the cracks may be the result of the difference in thermal expansion between the mullite coating and the silicon-containing substrate. FIG. 1 shows differences in the coefficient of thermal expansion (CTE) of mullite, silicon carbide (SiC) and silicon (Si). Thus, there is a need to provide coatings or layers to silicon-containing substrates that act at least as environmental barrier coatings having reduced cracks.
The present invention is based on the discovery that a modifier component can be added to a mullite coating to reduce cracks in the coating applied to a silicon-containing substrate. The mullite coating with the modifier component is also referred to as a modified mullite coating. The modified mullite coating reduces fracture at the interface of the mullite coating and the silicon-containing substrate.
In one aspect, the invention is an article comprising a silicon-containing substrate and a modified mullite coating. The modified mullite coating comprises mullite and a modifier component that reduces cracks, including through-thickness cracks, in the mullite coating. Preferably, the modifier component comprises a component having a lower thermal expansion than the mullite coating. As a result, the modifier component imparts a lower thermal expansion coefficient to the mullite coating. The article can further comprise an external environmental/thermal barrier coating applied to the modified mullite coating. The modified mullite coating then functions as a bond coat between the external environmental/thermal barrier coating and the silicon-containing substrate.
In another aspect, the invention relates to a method of forming an article with at least a modified mullite coating. In the method, a silicon-containing substrate is formed and a modified mullite coating is applied. The modified mullite coating comprises mullite and a modifier component that reduces cracks in the coating.